A targeting vector was designed to insert a CreERT2 fusion gene (Cre recombinase fused to a G400V/M543A/L544A triple mutation of the human estrogen receptor ligand binding domain), an SV40 polyA signal, and an frt-flanked neo cassette into the initiation codon of the cholecystokinin locus (Cck). This construct was electroporated into C57BL/6-derived embryonic stem (ES) cells. Correctly targeted ES cells were injected into recipient blastocysts and chimeric mice were bred with C57BL/6 mice to originate the colony. Mutant mice were bred with Actin-FLPe mice (on a C57BL/6 congenic background (N10)) to remove the neo selection cassette. These Cck-CreER (or Cck-CreERT2) mice were subsequently bred to C57BL/6 inbred mice for two additional generations and then bred together for many more generations (and the FLPe transgene was removed).